Ordinary and Extraordinary Hadrons

Resonances and enhancements in meson-meson scattering can be divided into two classes distinguished by their behavior as the number of colors (Nc) in QCD becomes large: The first are ordinary mesons that become stable as Nc →∞. This class includes textbook qq mesons as well as glueballs and hybrids. The second class, extraordinary mesons, are enhancements that disappear as Nc →∞; they subside into the hadronic continuum. This class includes indistinct and controversial objects that have been classified as qqqq mesons or meson-meson molecules. Ordinary mesons resemble Feshbach resonances while extraordinary mesons look more like effects due to potentials in meson-meson scattering channels. I build and explore toy models along these lines. Finally I discuss some related dynamical issues affecting the interpretation of extraordinary mesons.